Add preferences abstraction

[faern]

PR #8 was growing quite a lot. So I felt like splitting out some of the code for separate review in order to shrink that PR a bit.

This PR introduces the safe abstraction for SCPreferences. The first commit is the code from @LuoZijun that I cherry picked from the other PR branch. The second commit is where I add some simplifying constructors making it easier to construct the type for the most common use case.

@LuoZijun please add your feedback to this as well.

Please note that all constructors take &CFString instead of &str. This is in line with how we have done in dynamic_store.rs and also with how other crates implementing Apple APIs usually do. Example: https://docs.rs/core-graphics/0.13.0/core_graphics/font/struct.CGFont.html#method.from_name

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This change is 

[LuoZijun]

@faern I think your implementation is pretty good.

When this PR got merged, i will rebase PR-8.

[LuoZijun]

Link: https://developer.apple.com/documentation/systemconfiguration/scpreferences-ft8

[faern]

Review status: 0 of 2 files reviewed at latest revision, 2 unresolved discussions.

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system-configuration/src/preferences.rs, line 60 at r3 (raw file):

        unsafe {
            SCPreferences::wrap_under_create_rule(SCPreferencesCreate(

I fixed a bug here now where the reference count was wrong. Since wrap_under_get_rule was used the retain count was incremented when it should not have been. Added test as well.

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Comments from Reviewable

[faern]

Review status: 0 of 2 files reviewed at latest revision, 2 unresolved discussions.

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system-configuration/src/preferences.rs, line 13 at r2 (raw file):

Previously, LuoZijun (寧靜) wrote…

Link: https://developer.apple.com/documentation/systemconfiguration/scpreferences-ft8

Thanks. Done

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Comments from Reviewable

[pronebird]

Reviewed 1 of 2 files at r1, 1 of 1 files at r4.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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system-configuration/src/preferences.rs, line 32 at r4 (raw file):

impl SCPreferences {

Am I right that we do not implement any other methods except instantiation because we don't directly access them?

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Comments from Reviewable

[faern]

Review status: all files reviewed at latest revision, 3 unresolved discussions.

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system-configuration/src/preferences.rs, line 32 at r4 (raw file):

Previously, pronebird (Andrei Mihailov) wrote…

Am I right that we do not implement any other methods except instantiation because we don't directly access them?

Exactly. We don't need anything else for now. We just need access to the SCPreferencesRef in order to send into other functions. For now we will only need it for the SCNetworkService stuff etc.

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Comments from Reviewable

[pronebird]

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Review status: all files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable

[pronebird]

Reviewed 1 of 1 files at r5.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable